In the context of a mobile wireless communication network, the term “attachment” refers to the procedure whereby a user device connects to a local wireless network (such as a wireless LAN access point) and is able to make use of at least some of the services offered by that network. In practice, this procedure involves multiple protocol layers relating, for example, to the identification of the correct radio frequencies, radio-layer negotiation to enable communications with the access point, network access authentication and authorization procedures, link layer security protection initiation, finding the routers and addresses at the IP layer, and reestablishing mobility mechanisms to a new IP address. Unfortunately, these tasks take time to complete, and the interaction and overall effects of the individual tasks are not well understood, because most of the work on wireless access issues has focused only on a particular aspect.
An area likely to suffer in particular from a failure to inter-relate multiple protocol issues is that of mobility between different network types. For example, researchers in this area have tended to ignore the effects of having to have access control on the link (necessary due to business and/or legal requirements). Real users are only about to start taking advantage of mobility between different network types and the associated problems have therefore not been fully seen or appreciated.
Mobile IP is a set of protocols which provide for the roaming of subscribers between access networks, whilst at the same time ensuring that the subscribers are reachable by correspondent nodes that do not know the current locations of the subscribers. FIG. 1 illustrates schematically a network architecture for implementing Mobile IP. A subscriber 1 is attached to an access router 2 of an access network 3. Fundamental to Mobile IP is the provision of a Home Agent 4 in a subscriber's home network 5 and which knows the current location of the subscriber 1 (the current location being defined by an IP address known as a “care-of-address”) and is able to route messages directed to the subscriber's fixed IP address to the current location. Binding update messages are used to enable the subscriber 1 to update his care-of-address at the Home Agent 4, e.g. in the event that the subscriber roams to a new access network. When a subscriber changes its care-of-address, a route optimisation procedure may be invoked to ensure that packets subsequently sent from correspondent hosts 6 attached to respective access networks 7 are routed to the subscriber via the optimal route. An Authentication, Authorisation, and Accounting (AAA) server 8 located in the home network 5 communicates with the Home Agent 4.
In the case of Internet Protocol version 6 (IPv6), the process for network attachment in a typical wireless link is as follows:                Link layer attachment, such as detecting and connecting to a specific Wireless Local Area Network (LAN) access point.        Access control procedures. Mechanisms such as 802.1X and EAP are used for this. Typically, this involves three EAP control messages (identity request, response, and success, piggybacked on the EAPOL-Success message), and a specific authentication method. Simple authentication methods complete in two messages, but many methods require more.        Router Discovery. This is the process of finding the default router for the node and determining the routing prefixes for this link. In the simplest case this requires two messages, with a waiting period in between.        Duplicate Address Detection (DAD). This is used to ensure that the address that the mobile node selects for use on this link is unique. Typically, this involves one message and a waiting period.        Mobility management procedures. These include messaging with a Home Agent and possibly with correspondent nodes and a previous router. The messaging consists typically of two messages with the exchanged between the user terminal and the Home Agent, five (partially simultaneous) messages with each correspondent node, and a message with the previous router.        
Internet Protocol version 4 (IPv4) behaves largely in the same manner as IPv6. However, Router Discovery, Neighbour Discovery, and address autoconfiguration are replaced with the Dynamic Host Control Protocol (DHCP), and there is no support for DAD. DHCP typically requires four messages. Mobile IPv4 does not have route optimisation, and therefore involves only two additional mobility related messages. There is no support in IPv4 for a smooth handover from an old to a new access router.
In summary, with IPv6 there are at least 16 messages in the full case assuming only one correspondent node, and two distinct waiting periods (although four of the messages can be sent in parallel). In the IPv4 case, the number of messages is somewhat smaller due to the lesser functionality of IPv4 and the central role of DHCP. However, at least 11 messages are still needed.
Work is ongoing to try to optimise some of the signalling procedures discussed above. In particular:                So-called “Optimised” DAD attempts to avoid delays associated with DAD, and may also enable the use of the tentative address before DAD has completed. The potential benefit of this approach is the elimination of one waiting period, and possible additional parallelism in the messaging sequence. Another proposed approach uses the access router to assist in the DAD procedure.        Optimised Movement Detection attempts to make it faster to detect when movement (of a user terminal) has occurred, and to identify the network parameters in the new network. This involves new algorithms for the reduction of the waiting periods associated with IPv6 Router Advertisements, but does not reduce the overall amount of messages.        Hierarchical Mobile IP (HMIP) attempts to localise movements so that the number of location updates sent to the Home Agent and the correspondent nodes can be minimized.        
These optimisation approaches are mainly concerned with the elimination of unnecessary waiting times. They do not appear to have a significant impact on the amount of required signalling, with the except of HMIP. HMIP does not, however, reduce the amount of basic network access signalling, it only shortens the path that this signalling needs to take.